1. Field of the Invention
The present invention relates to satellite navigation systems, and, more particularly, to satellite navigation systems employing wireless communications technology to enhance signal-detection sensitivity.
2. Description of the Related Art
A satellite navigation system, such as the Global Positioning System (GPS), comprises a constellation of satellites that transmit GPS signals that can be used by a wireless terminal to determine the wireless terminal's position. The orbits of the satellites are arranged in multiple planes in order that signals can be received from at least four satellites at any position on earth. More typically, signals are received from six or eight satellites at most places on the earth's surface.
FIG. 1 illustrates GPS system 100 of the prior art. In prior art system 100, one or more satellites 101 of a satellite constellation transmit GPS signals 102 that are received by a wireless terminal 103. As is known in the art, the positioning operation is performed by receiving GPS signals 102 from three or more satellites. The basic method of determining position is based on knowing the time difference for each of the satellites. The time difference for a satellite is the time required for a GPS signal 102 initiated at the satellite to be received by wireless terminal 103. When GPS signals 102 from three satellites are simultaneously received, a “two-dimensional” position (latitude and longitude) can be determined. When GPS signals 102 are received from four or more satellites simultaneously, a “three-dimensional” position (latitude, longitude, and altitude) can be determined. Wireless terminal 103 typically comprises a GPS receiver 105 for receiving GPS signals 102 via a GPS antenna 107. A measured range, referred to as a “pseudorange,” is determined between GPS receiver 105 and each of the satellites based upon the corresponding signal propagation time. The measured range is referred to as the pseudorange, because there is typically a time-offset between the mutually synchronized clocks on the satellites and the local clock within GPS receiver 105. To determine a three-dimensional position, at least four GPS signals are needed to solve for the four unknowns representing the time offset and the three-dimensional position of wireless terminal 103.
The nature of the GPS signals transmitted from the satellites is well known in the art. Each satellite transmits two spread-spectrum, L-band carrier signals, referred to as the L1 and L2 signals. Two signals are needed if it is desired to eliminate errors that may arise due to refraction of the transmitted signals by the ionosphere. The L1 signal from each satellite is binary-phase shift-key (BPSK)-modulated by two pseudorandom noise (PN) codes in phase quadrature. A pseudorandom noise code sequence is a series of numbers that is random in the sense that knowledge of which numbers have already been received does not provide assistance in predicting the next received number. Further, using a binary PN code to modulate the phase of a carrier signal produces a suppressed carrier spread-spectrum signal. The L2 signal from each satellite is BPSK-modulated by only one of the PN codes. Use of the PN codes allows simultaneous use of a plurality of GPS signals for determining a receiver's position and for providing satellite-specific navigation information. A GPS signal transmitted by a particular satellite is selected by generating and matching, or correlating, the PN code for that particular satellite. Some of the PN codes are known and are generated or stored in GPS receiver 105. Other PN codes are not publicly known.
A first known PN code for each satellite is the “coarse acquisition” or C/A code. A second known PN code for each satellite is the “precision” or P code. The C/A code is intended to facilitate rapid GPS signal acquisition and hand over to the P code. The C/A code is a relatively short, coarse-grained code. The C/A code has a relatively short length before it repeats. The P code, on the other hand, is a relatively long, fine-grained code. The full P code has a length of 259 days, with each satellite transmitting a unique portion of the full P code. The portion of the P code transmitted by a given satellite has a length of precisely one week before the portion repeats.
In GPS receiver 105, signals corresponding to the known C/A and P codes may be generated in the same manner as in the satellites. The L1 and L2 signals from a given satellite are demodulated by aligning the phases, i.e., adjusting the timing of the locally generated codes with those received from the satellites. To achieve phase alignment, the locally generated code replicas are correlated with the received GPS signals until the resultant output signal reaches a peak. Because the time at which each particular bit of the PN code sequence is transmitted from the satellite is defined, the time of receipt of a particular bit can be used as a measure of the range to the satellite. Because the CIA and P codes are unique for each satellite, a specific GPS satellite may be identified based on the results of the correlations between the received GPS signals and the locally generated C/A and P code replicas. Methods for generating the C/A and P codes are set forth in various publicly available publications.
The C/A code component of the L1 signal is provided for commercial use. Various techniques have been developed to replicate the C/A code in GPS receivers. As a consequence of the repetition of the C/A code approximately once every millisecond, correlation at the GPS receiver may be performed in the absence of precise knowledge of the time of transmission of each C/A code bit. Acquisition of the P code is generally acquired by first locking onto the C/A code. Once the C/A code has been acquired, the C/A code-modulated carrier component of the L1 signal carrier alone may allow for satisfactory measurements. However, when high-resolution measurements are desired to be made quickly, the L2 carrier signal must also be used. The unknown atmospheric delay of the L1 and L2 carriers may be determined when both the L1 and L2 carriers are used. The GPS signals are intended to be recovered by correlating each incoming signal with a locally generated replica of each code, both the P code and the C/A code. The result of such correlation is that the carrier in the GPS signals is totally recovered when the modulating signal is the PN code sequence that matches the P code or the C/A code. The locally generated PN code is adjusted in time to provide an optimum correlation with the incoming signal. The correlation output is then a single narrowband peak centered at the carrier frequency. The carrier recovered by correlation provides the best available signal-to-noise ratio.
The key to achieving GPS navigational performance is the processing of one or more GPS signals, each of which carries a coarse acquisition code, to achieve code acquisition of these GPS signals.
There are two commonly known approaches for the C/A code acquisition process: code acquisition in the frequency domain and code acquisition in the time domain. Frequency-domain code acquisition schemes are based on the fast Fourier transformation. Time-domain code acquisition schemes are processes that align the incoming GPS code with the local replicated code within one chip or, equivalently about one micro-second, since the chip rate for a GPS C/A code is 1.023 MHz.
For a conventional wireless terminal to achieve code acquisition in the time domain, the correlation procedure is typically implemented many times, where each implementation searches one possible code phase at one possible Doppler frequency. There are a total of 1023 chips in a C/A PN code, and the Doppler frequency range of the GPS signal is (−5000, +5000) Hz. As a result, the maximum number of code-phase searching steps is 2046, while the maximum number of Doppler searching steps is 200. For each searching step, one may increase code phase by a half-chip, or one may increase the Doppler frequency by 500 Hz. As a result, the acquisition time is proportional to the product of the number of code-phase searching steps and the number of carrier-frequency searching steps.
If the satellite Doppler frequencies are known or can be predicted, they can be utilized to reduce the signal detection acquisition time for both the time-domain and the frequency-domain approaches. If the satellite code phases or ranges are known or can be predicted, they may be useful for the time-domain code acquisition schemes provided that the wireless terminal has built a precise timing reference. The frequency-domain code acquisition scheme however does not rely on the satellite code phase information and thus places a heavier computational burden on the wireless terminal than the time-domain code acquisition approach. Furthermore, the wireless terminal utilizing a frequency-domain code acquisition scheme takes a longer time to fund its position.